Wastewater treatment generally gives rise to sewage sludge (primary, activated and tertiary sludge) which contains in concentrated form the majority of the ingredients eliminated from the wastewater. This comprises plant nutrients such as phosphorus and nitrogen, and also organic and inorganic ingredients. To stabilize and to reduce the volume of sewage sludges, anaerobic treatment is performed in digestion vessels. During this sludge digestion, the degradation of organic substrate releases not only the digester gas which arises, but also organically bound nitrogen and phosphorus compounds. This method and the removal of phosphate are described in “Auswirkungen der thermischen Klärschlammhydrolyse und der prozessintegrierten Nährstoffrückgewinnung auf die Stoffstrom-und Energiebilanzen auf Kläranlagen” (“Effects of thermal sewage sludge hydrolysis and of process-integrated nutrient recovery on the material-flow and energy balances on sewage plants”), final report AZ 24507-23, H. Bormann, M. Sievers, W. Ewert, published by Deutsche Bundesstiftung Umwelt (German Federal Foundation for the Environment), 2010 (http://www.dbu.de/projekt_24507/_db_1036.html). The removal of phosphate from the wastewater is achieved in practice by chemical precipitation with iron/aluminum salts or by increased biological P uptake into microorganisms (Bio-P). To increase the phosphorus recovery potential, thermal hydrolysis of surplus sludge and subsequent separate digestion of the surplus sludge is proposed. During the thermal hydrolysis, there is a partial redissolution of phosphorus and nitrogen from the solids of the surplus sludge. Phosphate recovery is achieved by phosphate precipitation of the redissolved phosphorus portion in the anaerobically treated surplus sludge.
D. Stumpf: “Phosphor Recycling durch MAP-Fällung im kommunalen Faulschlamm” (“Phosphorus recycling by MAP precipitation in communal digester sludge”), Umweltbundesamt Berlin (Federal Environmental Agency in Berlin), 2007 describes various methods and appliances for removing phosphorus by precipitating magnesium ammonium phosphate (MAP) from sewage sludge. Traditionally, this is carried out after the anaerobic treatment of sewage sludge, i.e., surplus sludge and/or primary sludge. The MAP precipitation is generally carried out using precipitation reactors or fluidized-bed reactors in which the pH is increased by addition of alkaline solution or by air stripping. MAP is traditionally precipitated directly from the digester sludge or from process water of sludge dewatering. The precipitated MAP can then be removed by means of sieves, hydrocyclone or centrifuges. In some cases, it then has to be further processed in order to be able to be used as, for example, fertilizer.
The methods carried out after completed digestion process in order to remove magnesium ammonium phosphate (MAP) prevent undesired crystallizations, which can frequently lead to operational interruptions. Here, advantage is taken of the known fact that, after use of biological phosphate elimination, the majority of the stored polyphosphates in the surplus sludge are redissolved under anaerobic conditions as orthophosphates in exchange for organic acids. This effect is utilized in the recovery of phosphates by the phosphates being precipitated and removed as magnesium ammonium phosphate (MAP) in either the digested sludge or the separated sludge water. The magnesium source used is generally magnesium chloride, magnesium oxide or magnesium hydroxide.
In the case of the redissolution of phosphates from the surplus sludge or raw sludge, preference is given to the phosphate exchange processes which occur under anaerobic conditions. The disadvantage here is that it is necessary to work with relatively large volumes and that relatively low phosphate concentrations are obtained.
EP 1 364 915 A1 discloses a method for reducing phosphate from sewage sludge. In said method, wastewater is fed to aerobic treatment after the anaerobic treatment and sludge recycled from a settling tank is subjected to anaerobic treatment. The liquid phase is then fed to an apparatus for removing phosphate, for example a MAP reactor.
WO 2009/112208 A2 discloses a method for wastewater treatment and a wastewater treatment plant for this purpose, in which hydrolyzed and subsequently anaerobically treated surplus sludge is fed to a precipitation unit in order to remove phosphate. There, magnesium ammonium phosphate (MAP) is precipitated from the processed hydrolyzed and anaerobically treated surplus sludge by addition of magnesium salts with the setting of an appropriate pH of from 7.5 to 7.8.